The conventional sound source is mostly two-channel stereo audio. Therefore, a two-channel-wise audio processing is implemented in the case of a audio processing apparatus implementing level corrections of the conventional audio data. More specifically, volume levels are detected with respect to an inputted two-channel audio data and a level correction processing is implemented in accordance with the volume levels to thereby complete the level corrections of the audio data. This method is implemented by means of, for example, a audio processing apparatus shown in FIG. 17.
A conventional audio processing apparatus 5000 comprises a level judging unit 5100 and a level correcting unit 5200. The level judging unit 5100 judges whether the volume levels of the audio data inputted from outside are larger or smaller than a given reference volume. The level correcting unit 5200 implements the level corrections according to the volume levels judged by the level judging unit.
The level judging unit 5100 retrieves one sample data for each channel from the inputted two-channel audio data, and compares the retrieved data from the two channels to thereby employ the larger value as a level detection data. The level judging unit 5100 then compares the level detection data with the reference volume serving to judge the volume levels, and transmits the comparison results in the form of level judgment result signals to the level correcting unit 5200.
The level correcting unit 5200, based on the level judgment result signals from the level judging unit 5100, reduces the volumes when the inputted data is larger than the reference volume and increases the volumes when the inputted data is smaller than the reference volume. A smoothing processing is implemented between the output results in consequence of the foregoing processings and output data of a previous sample so that the smoothing-processed output is handled as output data of a current sample.
Such a audio processing apparatus is used for a vehicle-mounted audio apparatus and an audiovisual apparatus used in a limited volume at midnight. The audio processing apparatus, in the foregoing uses, can compress a dynamic range of the two-channel audio data, thereby enabling a small sound to be more easily heard. Therefore, the audio apparatus can be fully enjoyed in noise-penetrating environments such as a car interior and any situation where the sound volume must be reduced.
The apparatus employing the conventional audio processing is disclosed in No. 5-275950 of the publication of unexamined patent applications. According to the conventional technology, when the dynamic range of sound signals are compressed in the case of CD (compact disc) reproduction and the like, a property of excessive compression can be improved without deteriorating a distortion rate, and the dynamic range can be thereby normally compressed without any distortion.
In the case of processing DVD, which is rapidly spreading in recent years, and the like, it is necessary to process multi-channel audio data. When the conventional audio processing apparatus 5000 is applied to the multi-channel audio data, a possible method is to down-mix the multi-channel audio data into the two-channel one, which is thereafter inputted to the audio processing apparatus 5000. However, the audio processing apparatus 5000 can only emit the two-channel output in spite of the multi-channel input source. On top of that, there is a problem as follow.
The audio data has been increasingly diversified in accordance with the evolution of the multi-channel audio data. It can be hardly said to be flexibly responding to users' needs for the audio processing if the audio processing is implemented to all the sound sources at a universally common standard. In the case of the two-channel stereo sound source, it is not particularly necessary to change settings between the right and left channels. On the contrary, in the Dolby Digital, for example, five channels in total, which are a left front channel, right front channel, center channel, and surround two channels, and LFE (Low Frequency Effect) recording low-frequency sound effects are independently recorded and reproduced without being blended with one another. In such a case, an optimum setting is diverse depending on the sound sources, for example, the center channel should be pealed most, the front channel should be emphasized, LFE should be emphasized or the like. The multiple settings are even further diversified due to the users' needs. However, it is difficult to optionally change the settings in the configuration where the audio data is down-mixed into the two channel data.